1. Technical Field
The present application relates to laser diodes, and more particularly to asymmetric distributed Bragg reflector configuration for single mode operation.
2. Description of Related Art
Transmission of light through waveguides has been pursued for many types of communications applications. Light signals offer many potential advantages over electronic signals. Light sources are commonly created from semiconductor devices, and include semiconductor devices such as LEDs (Light Emitting Diodes) and LDs (Laser Diodes).
Optical fiber is the most commonly used transmission medium for light signals. A single fiber is capable of carrying several different modulated signals within it at one time. For instance, wavelength division multiplexing divides the used bandwidth of the fiber into different channels (each channel containing a small range of wavelengths) and thus transmits several different wavelengths (or signals) of light at once. Using such a system requires sources for the different wavelengths. More wavelengths on the fiber require more sources to be coupled to the fiber.
Efficient coupling of light into a fiber is simplified if the laser beam has a cross sectional profile that matches the profile of the fiber mode(s). Efficient use of light for communications requires that the light have high temporal coherence. Efficient coupling of light to monomode guides requires spatial coherence. Spatial coherence requires the laser to operate in a single lateral and transverse mode. Temporal coherence requires the laser to operate in a single longitudinal mode and implies a very narrow bandwidth, or range of wavelengths.
The most coherent semiconductor lasers use resonators based on grating feedback rather than Fabry-Perot resonators with reflective end facets. Distributed feedback (DFB) lasers use a Bragg reflective grating covering the entire pumped length of the laser. An alternative to DFB lasers is the use of distributed Bragg reflectors (DBRs) located outside the pumped region.
In conventional DFB and DBR lasers, light is removed through an end facet and the output beams have dimensions entirely controlled by the vertical (i.e., normal to the surface) (x) and lateral (y) size and the composition of the guiding structure. Such output beams typically have too great a divergence for effective coupling to optical fibers, or for other applications requiring beams with low divergence angles.
Beam dimensions (in at least one direction) larger than that available from laser facets may be obtained by using a Bragg grating to couple light out of the waveguide normal (or at certain fixed angles) to the waveguide surface. So called second order Bragg gratings have a period equal to the wavelength of light of the guided mode. The second grating order of such a grating reflects some of the light back in the waveguide plane while the first order couples some of the light normal to the plane. So called first order (Bragg) gratings have a period equal to one half the wavelength of light in the guided mode, reflect light in the waveguide plane, and do not couple light out of the waveguide. First, second, and third order (etc.) gratings are sometimes referred to as being in resonance. A non-resonant grating couples light out of the waveguide at an angle to the normal and does not reflect any light in the waveguide plane.
U.S. Pat. No. 5,970,081 to Hirayama et al. appears to show a laser with a distributed feedback (DFB) grating of second order or higher that claims to obtain a Gaussian shaped output beam by narrowing the waveguide or using a chirped grating at the outcoupling portion. They do not seem to recognize that by so doing the resonant wavelength of the grating is altered along the length of the narrowing or chirping. This would be expected to result in an output which will fan in angle along the longitudinal direction rather than produce a simple Gaussian intensity variation emitted normal to the plane as claimed. They do not define the beam shape in the lateral direction. In all versions they choose second order outcoupling gratings which, absent a narrowing waveguide or chirp, would emit light perpendicular to the surface of the laser waveguide.
U.S. Pat. No. 4,006,432 to Streifer et al. appears to show a grating out-coupled surface emitting DFB laser. The grating period may be chosen to be either resonant or not.
A paper by Bedford, Luo, and Fallahi titled Bow-Tie Surface-Emitting Lasers (IEEE Photonics Technology Letters, Vol. 12, No. 8, August 2000) appears to show a DBR laser with curved second order grating at the ends to couple light out of the waveguide. The same gratings are used for outcoupling and for reflecting the light within the waveguide. They mention the use of non-resonant gratings in conjunction with reflector gratings if emission at other than the direction normal the waveguide plane is desired. The paper appears to show a flared resonator region which allows symmetric outcoupling from both ends of the laser. This facilitates two outputs that are coherent with one another, with emission in the same direction. Such a device is meant to alleviate the complications of controlling the relative phase between the two emitters.
The Tiberio article (Facetless Bragg reflector surface-emitting AlGaAs/GaAs lasers . . . , J. Vac. Sci. Technol., B9(6), 1991) appears to show a surface emitting laser diode that uses first order reflective gratings and either second order (or non-resonant) gratings for outcoupling. Thus, depending on the chosen grating period, the outcoupled beam can be normal or at an angle to the surface.
U.S. Pat. No. 6,064,783 to Congden appears to show a DBR laser with a grating assisted waveguide coupler that couples light from the laser waveguide to a parallel fiber-like glass waveguide for later coupling to a fiber. Several different lasers are coupled to similar fiber-like glass waveguides in the figures. The fiber axis is parallel to the laser waveguides. This reference mentions that this model is easily attached to a fiber through xe2x80x9cbutt coupling.xe2x80x9d The grating acts as a Quasi Phase Matching element to couple the light from the laser waveguide to the fiber-like glass waveguide.
Symmetric, grating-outcoupled, surface emitting lasers (GO-SELs) employ a configuration with shallow, long, narrow-band DBRs at both ends of the laser cavity, with an outcoupling grating in the middle. For symmetric devices, the reflectivity spectrum of both DBRs must overlap for lasing to occur. However, this overlap is much harder to achieve during wafer processing in symmetric designs because of process variations between the DBRs. In symmetric GO-SELs, both DBRs have very narrow reflective spectrum widths. Limitations in the manufacturing process usually produce slight reflectivity variations between the two DBRs, thus preventing both DBRs from having exactly the same reflective spectrum width. This slight variation in reflective spectrum widths results in two laser wavelengths, rather than one, thus reducing light coherence. In addition, temperature variations can also reduce laser coherence by causing changes in the reflectivity spectrum of the DBRs.
Therefore, it would be desirable to have a GO-SEL design that allowed for single mode operation, without the need to exactly match narrow DBR reflective spectrum widths.
The present invention provides a laser source comprising a laser diode and has an active region with distributed Bragg reflectors (DBRs) at either end to reflect light within the cavity, and an outcoupling grating in the center of the device, which couples light out of the cavity. On either side of the outcoupling grating is a gain region with electrical contacts for supplying current. In some embodiments, the outcoupling grating may be at the end of a gain region with only a DBR and not a gain region on the other side.
The DBRs are asymmetric. One DBR is long and shallow, with a narrow-band reflective spectrum. The other DBR is short and deep, with a wide-band reflective spectrum. The lasing wavelength is determined by the reflective spectrum overlap of the two DBRs. Since the shallow DBR is highly reflective to only one Fabry Perot wavelength, and the deep DBR is highly reflective to a wide band of Fabry-Perot wavelengths, it is the reflective spectrum of the shallow DBR that determines the lasing wavelength. This allows the asymmetric device to produce a single-wavelength laser without the need to match the reflective spectrum widths of the DBRs.